This application discloses an invention that is related, generally and in various embodiments, to systems and methods for measuring a colored flexible material during a manufacturing process.
For printing processes such as sheet-fed lithographic printing, flexographic, ink jet, etc., it is very important to accurately monitor the output of the production equipment for quality control purposes. A common example of production equipment that produces colored flexible materials is a sheet-fed printing press. In general, as flexible sheets advance through the press, colors are applied to the flexible sheets at various printing units of the press. The colored flexible sheets may then be guided through additional paths of the production process. For example, the colored flexible sheets may be guided through coating and drying stages of the production process. Typically, the final step of the production process is to collect all of the colored flexible sheets in a delivery unit positioned at an end of the printing press. For such processes, the color of the flexible printed materials is usually monitored in one of two ways.
The first method involves having the press operator remove one of the recently printed sheets from the delivery unit, carry the sheet over to a flat surface that is often positioned on or near the press console, and lay the sheet down onto the flat surface. A color controlled light positioned above the flat surface allows the press operator to visually inspect the printed colors and to measure the sheet in either an automated or manual fashion with a spectrophotometer or densitometer. As this method requires press operator intervention, the process is not automated. In addition, the time delay between removal of the printed sheet from the delivery unit and the actual acquisition of the color data can vary considerably. Due to the ink dry back effect, the color of the printed sheets can change significantly in the first minutes after printing. Therefore, the variable time delay makes it difficult for someone using this method to accurately predict the final color of the printed sheets.
The second method is an automated method that allows for measurement of the printed sheet on the printing press. This method utilizes color measurement equipment that is integral with the printing press or one of the extra units. The printed sheets are measured at press speed, so the equipment must measure very quickly to sample a single sheet, or it must synchronize itself so that it can gather a complete dataset from many different pages. The color measurement equipment may operate in different ways. For example, the color measurement equipment may utilize flash lamps that can measure very quickly, in essence “stopping the motion” of the printing press. Alternatively, the color measurement equipment may utilize a slow scan across the printing press and synchronize the acquisition time to generate a complete dataset. This method measures the printed sheet within seconds of printing before the sheet arrives at the delivery unit. At the time of the measurement, the ink on the printed sheet will either be wet, or hot if a drying unit is used. In either the wet or dry condition, the ink color will be different from its long term “dried back” value. Although this method addresses the variable time delay problem associated with the first method, the cost of the system (i.e., the printing press system with the integral color measurement equipment) tends to be relatively expensive.